Background: An assessment of mental capacity includes an evaluation of the ability to "weigh up" information, but how to do this is uncertain. We have previously used a laboratory decision-making task, temporal discounting, which involves a trade-off between the value and the delay of expected rewards. Participants with intellectual disabilities (ID) showed very little evidence of "weighing up" of information: only a third of participants showed consistent temporal discounting performance, and when present, consistent performance was usually impulsive; and the ability to perform consistently was more strongly related to executive functioning than to IQ. The aim of the present study was to replicate these observations and extend them to a more realistic financial decision-making task. Methods: We administered a temporal discounting task and a financial decision-making task, as well as tests of executive functioning and IQ, to 20 participants who attended day services for people with learning disabilities (mean Full-Scale IQ = 59), and to 10 staff members. Results: Performance in both decision-making tasks was related more strongly to executive functioning than to IQ. In both tasks, decisions by service users were made largely on the basis of a single item of information: there was very little evidence in either task that information from two sources was being "weighed". Conclusions: The results suggest that difficulty in "weighing up" information may be a general problem for people with ID, pointing to a need for psycho-educational remediation strategies to address this issue. The importance of executive functioning in decision-making by people with ID is not recognized in the legal test for mental capacity, which in practice includes a possibly irrelevant IQ criterion.